1. Field
The present technology relates to semiconductor devices.
2. Description of Related Art
The strong growth in demand for portable consumer electronics is driving the need for high-capacity storage devices. Non-volatile semiconductor memory devices, such as flash memory storage cards, are becoming widely used to meet the ever-growing demands on digital information storage and exchange. Their portability, versatility and rugged design, along with their high reliability and large capacity, have made such memory devices ideal for use in a wide variety of electronic devices, including for example digital cameras, digital music players, video game consoles, PDAs and cellular telephones.
While a wide variety of packaging configurations are known, flash memory storage cards may in general be fabricated from so-called 3-D semiconductor devices, including for example a system-in-a-package (SiP) or a multichip module (MCM), where a plurality of die are mounted on a substrate in a stacked configuration. Edge views of conventional 3-D semiconductor packages 20 (without molding compound) are shown in prior art FIGS. 1 and 2. Typical packages include a plurality of semiconductor die 22 mounted to a substrate 26. In the examples shown, the die stack has four die, 22a, 22b, 22c and 22d. Further examples have more or less die in the stack. Although not shown in FIGS. 1 and 2, the semiconductor die 22 are formed with die bond pads on an upper surface of the die. Substrate 26 may be formed of an electrically insulating core sandwiched between upper and lower conductive layers. The upper and/or lower conductive layers may be etched to form conductance patterns including electrical leads and contact pads. Wire bonds 30 are soldered between the die bond pads of the semiconductor die 22 and the contact pads of the substrate 26 to electrically couple the semiconductor die to the substrate. The electrical leads on the substrate in turn provide an electrical path between the die and a host device. Once electrical connections between the die and substrate are made, the assembly is then typically encased in a molding compound to provide a protective package.
Semiconductor die are batch processed from silicon wafers. A wafer includes an active surface, in which the integrated circuits of the semiconductor die are formed, and an inactive surface, or backside, opposite the active surface. During fabrication, impurities are introduced into semiconductor wafers which can degrade the performance of the finished semiconductor die and reduce yield of acceptable devices. At elevated temperatures, these impurities become more mobile and tend to gravitate toward stress concentration areas in a wafer where they become trapped. It is therefore known to create stress concentration areas away from the active surface of the wafer, in a process known as gettering.
There are two broad categories of gettering: intrinsic gettering and extrinsic gettering. Intrinsic gettering is achieved by providing gettering sites within the interior of the wafer, away from the active surface. Some commonly known approaches include providing oxygen precipitates within the bulk silicon that serve as gettering sites. Extrinsic gettering is a process whereby dislocations and crystal disorders are created on the inactive surface of the wafer. These intentionally created disorders result in stress concentration areas which act as effective traps for the impurities to keep them away from the active surface of the semiconductor wafer.
One problem with known extrinsic gettering processes is that the creation of dislocations and stress concentrations is poorly controlled. As the thicknesses of semiconductor wafers are getting smaller and smaller, stress concentrations formed by conventional extrinsic gettering processes can result in a weakening of the wafer to the point of cracking or other mechanical failure of the wafer. Coming at the end of the wafer fabrication process, such failure has a high cost and can result in a significant reduction in yield.